Hydrogen storage alloy, hydrogen storage alloy electrode, secondary battery, and method for producing hydrogen storage alloy

ABSTRACT

Provided is a hydrogen storage alloy which is characterized in that two or more crystal phases having different crystal structures are layered in a c-axis direction of the crystal structures. The hydrogen storage alloy is further characterized in that a difference between a maximum value and a minimum value of a lattice constant a in the crystal structures of the laminated two or more crystal phases is 0.03 Å or less.

TECHNICAL FIELD

The invention relates to a hydrogen storage alloy, a hydrogen storage alloy electrode, a secondary battery, and a method for producing a hydrogen storage alloy.

BACKGROUND ART

A hydrogen storage alloy is an alloy capable of safely and easily storing hydrogen as an energy source and has drawn an attention as a new energy conversion and storage material and its application fields are in a wide range, e.g., hydrogen storage and transportation, heat storage and transportation, heat-mechanical energy conversion, separation and purification of hydrogen, isolation of hydrogen isotopes, batteries using hydrogen as active materials, catalysts for synthetic chemical, temperature sensors, and so forth.

For example, a nickel-metal hydride battery using a hydrogen storage alloy as a negative electrode material has characteristics such as (a) high capacity; (b) durability to overcharge and over discharge; (c) capability of charging and discharging at high efficiency; (d) cleanness and has been actively investigated to have further improved capabilities (improvement of retention ratio of capacity in the case of repeating charge and discharge, that is, cycle life, improved capacity of the battery, etc.).

So far, an AB₅ type rare earth-Ni-based alloy having a CaCu₅ type crystal structure has been put into practical use as an electrode material for a nickel-metal hydride battery, one application example of such a hydrogen storage alloy; however the discharge capacity reaches almost a limit of about 300 mAh/g and presently it becomes difficult to further improve the capacity.

Further, as a new hydrogen storage alloy, a rare earth metal-Mg—Ni based alloy, for example, LaCaMgNi₉ alloys (Patent Document 1) having a PuNi₃ type crystal structure have drawn attention and it is reported that a discharge capacity exceeding that of an AB₅ type alloy can be obtained by using these alloy for electrode materials.

It is also reported that in addition of the crystal phase having the AB₅ type crystal structure, electrode materials of hydrogen storage alloys containing a crystal phase of AB₂ type crystal structure such as MgCu₂ type or rare earth metal-Mg—Ni type alloys containing, as a main phase, the crystal phase having Ce₂Ni₇ type, CeNi₃ type, or Gd₂Co₇ type crystal structure have high hydrogen storage capacities and show good hydrogen release characteristics (Patent Document 2).

Furthermore, with respect to alloys having Ce₅CO₁₉ type crystal structure, it is reported that electrodes complexed with rare earth-Ni alloy having a CaCu₅ type crystal structure are excellent in hydrogenation reaction speed (Patent Document 3).

Patent Document 1: Japanese Patent No. 3015885

Patent Document 2: Japanese Patent Application Laid-Open (JP-A) No. 11-323469

Patent Document 3: Japanese Patent No. 3490871

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

However, in the above-mentioned conventional hydrogen storage alloys, a problem that the hydrogen storage capacity is decreased in the case where hydrogen storage and release are repeated has not sufficiently been solved yet.

Further, with respect to the conventional hydrogen storage alloys, there is another problem that when the hydrogen storage alloys are made to be excellent in the cycle life to quickly release the stored hydrogen, the stored hydrogen is gradually released simply by leaving the hydrogen storage alloys as they are.

In view of the above state of the art, one aim of the invention is to provide a hydrogen storage alloy of which the hydrogen storage capacity is hardly decreased even if hydrogen storage and release are repeated, that is, a hydrogen storage alloy excellent in the cycle life.

Another aim of the invention is to provide a hydrogen storage alloy excellent in the cycle life and having high hydrogen storage amount.

Further, another aim of the invention is to provide a hydrogen storage alloy having little self-release of hydrogen while maintaining the excellent cycle life.

Means for Solving the Problems

To solve the above-mentioned problems, the inventors of the invention have made various investigations and accordingly have found that a hydrogen storage alloy having a layered structure of a plurality of crystal phases with different crystal structures such as a crystal phase of Pr₅Co₁₉ type crystal structure, a crystal phase of Ce₂Ni₇ type crystal structure, and the like can exhibit remarkably excellent cycle life and the finding now leads to completion of the invention.

That is, the invention provides the following hydrogen storage alloy, a hydrogen storage alloy electrode containing the hydrogen storage alloy, a secondary battery comprising the electrode, and a method of producing the hydrogen storage alloy.

(1) A hydrogen storage alloy containing two or more crystal phases having different crystal structures which are layered in the c-axis direction of the crystal structures.

(2) The hydrogen storage alloy according to the above description (1) in which the difference of the maximum value and the minimum value of the lattice constant a in the crystal structures of the layered two or more crystal phases is 0.03 Å or less.

(3) The hydrogen storage alloy according to the above description (1) or (2) in which the crystal phases include two or more types selected from a group consisting of a crystal phase having La₅MgNi₂₄ type crystal structure, a crystal phase having Ce₅Co₁₉ type crystal structure, and a crystal phase having Ce₂Ni₇ type crystal structure.

(4) The hydrogen storage alloy according to one of the above descriptions (1) to (3) having a composition defined by a general formula R1_(a)R2_(b)R3_(c) (wherein R1 is one or more kind elements selected from a group consisting of rare earth metals including Y; R2 is one or more kind elements selected from a group consisting of Mg, Ca, Sr, and Ba; R3 is one or more kind elements selected from a group consisting of Ni, Co, Mn, Al, Cr, Fe, Cu, Zn, Si, Sn, V, Nb, Ta, Ti, Zr, and Hf; and a, b, and c satisfy 10≦a≦30; 1≦b≦10; 65≦c≦90; and a+b+c=100).

(5) The hydrogen storage alloy according to the above description (1) having a composition defined by a general formula R1_(d)R2_(e)R4_(f)R5_(g) (wherein R1 is one or more kind elements selected from a group consisting of rare earth metals including Y; R2 is one or more kind elements selected from a group consisting of Mg, Ca, Sr, and Ba; R4 is one or more kind elements selected from a group consisting of Ni, Co, Cr, Fe, Cu, Zn, Si, Sn, V, Nb, Ta, Ti Zr, and Hf; R5 is one or two elements selected from Mn and Al; and d, e, f, and g satisfy 8≦d≦19; 2≦e≦9; 73≦f≦79; 1≦g≦4; and d+e+f+g=100) and satisfying 3.53≦(B/A)≦3.80 and 0.0593(B/A)+1.59≦rA≦0.0063(B/A)+1.81 in the case (B/A) is defined as (f+g)/(d+e) and rA (Å) is defined as the average atom radius of the R1 and R2.

(6) The hydrogen storage alloy according to the above description (5) in which R1 consists of one or more kind elements R1′ selected from a group consisting of Ce, Pr, Nd, Sm, and Y and La and the ratio of La/R1′ is 5 or less; the R2 is Mg; the R4 is one or two elements selected from Ni and Co; the R5 is Al; and d, e, f, and g satisfy 16≦d≦19; 2≦e≦5; 73≦f≦78; and 2≦g≦4.

(7) The hydrogen storage alloy according to the above description (5) or (6) having a main generative phase is a crystal phase having Pr₅Co₁₉ type crystal structure or a crystal phase having Ce₅Co₁₉ type crystal structure.

(8) The hydrogen storage alloy according to the above description (1) having, as a main generative phase, a crystal phase having Ce₅Co₁₉ type crystal structure and a composition defined by a general formula La_(h)R6_(i)R7_(j)Mg_(k)R8_(m) (wherein R6 is one or more kind elements selected from a group consisting of rare earth metals including Y and excluding La; R7 is one or more kind elements selected from a group consisting of Zr, Ti, Zn, Sn and V; R8 is one or more kind elements selected from a group consisting of Ni, Co, Mn, Al, Cu, Fe, Cr, and Si; and h, i, j, k and m satisfy 0≦j≦0.65; 2≦k≦5.5; 0.70≦h/(h+i)≦0.85; and h+i+j+k+m=100).

(9) The hydrogen storage alloy according to the above description (1) in which the ratio of the crystal phase having CaCu₅ type crystal structure is 22% by weight or less.

(10) The hydrogen storage alloy according to the above description (9) in which the hydrogen equilibrium pressure is 0.07 MPa or less.

(11) The hydrogen storage alloy according to the above description (9) or (10) having a composition defined by a general formula R1_(n)R2_(p)R4_(q)R5_(r) (wherein R1 is one or more kind elements selected from a group consisting of rare earth metals including Y; R2 is one or more kind elements selected from a group consisting of Mg, Ca, Sr, and Ba; R4 is one or more kind elements selected from a group consisting of Ni, Co, Cr, Fe, Cu, Zn, Si, Sn, V, Nb, Ta, Ti, Zr, and Hf; R5 is one or two kind elements selected from Mn and Al; and n, p, q, and r satisfy 16≦n≦23; 2≦p≦8; 68.5≦q≦76; 1≦r≦6.5; and n+p+q+r=100).

(12) The hydrogen storage alloy according to one of the above descriptions (9) to (11 ) wherein the content of Mn is 5% by weight or less.

(13) A hydrogen storage alloy electrode using the hydrogen storage alloy according to any one of the above descriptions (1) to (12) as a hydrogen storage medium.

(14) A secondary battery using the hydrogen storage alloy electrode according to the above description (13) as a negative electrode.

(15) A method for producing the hydrogen storage alloy according to one of the above descriptions (1) to (12), comprising a melting step of heat melting alloy raw materials at prescribed mixing ratio in inert gas atmosphere; a cooling step of rapid solidification the melted alloy at a cooling speed of 1000° C./s or higher; and an annealing step of further annealing the alloy subjected to the cooling step at 860° C. or higher and 1000° C. or lower in inert gas atmosphere in pressurized state.

Herein, crystal phase in the invention means a region where a single crystal structure exists.

Effect of the Invention

With respect to conventional hydrogen storage alloys, some contain two or more crystal phases having crystal structures different from one another. However, unlike the invention, these crystal phases are not layered in the c-axis direction and exist independently in individual regions. Therefore, it is supposed that significant lattice strains are caused in the respective crystal phases at the time of absorption and release of hydrogen are repeated and as a result, deterioration of the alloys such as pulverization is caused when absorption and release of hydrogen are repeated, resulting in aggravation of the cycle life.

On the other hand, with respect to the hydrogen storage alloy of the invention, two or more crystal phases having crystal structures different from one another are layered in the c-axis direction of the crystal structures. Therefore, the strains of the crystal phases caused because of repeated absorption and release of hydrogen are remarkably moderated. Such moderation of the strains suppresses deterioration at the time of repeating absorption and release of hydrogen and as a result, the cycle life can remarkably be improved.

Accordingly, the hydrogen storage alloy of the invention is provided with an effect to cause an excellent cycle life. Further, since the hydrogen storage alloy electrode and the secondary battery of the invention are configured by using such a hydrogen storage alloy, even if discharge and charge are repeated, they are provided with an excellent property that the discharge capacity is hardly decreased. Furthermore, the method for producing the hydrogen storage alloy of the invention is provided with an effect to efficiently produce such a hydrogen storage alloy.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1: A schematic drawing showing one embodiment of a first hydrogen storage alloy.

FIG. 2: A TEM image showing one example of the first hydrogen storage alloy.

FIG. 3: A magnified drawing of a portion of FIG. 2.

FIG. 4: A photograph showing the distribution of Ni and Mg obtained by EPMA, with respect to Example 1 and Comparative Example 1.

FIG. 5: A graph showing the measurement results of cycle life, with respect to hydrogen storage alloys of Example 1 and Comparative Example 1.

FIG. 6: A graph showing a relation of the difference of the a-axis length and the capacity retention ratio, with respect to the hydrogen storage alloys of Examples 2 and 6.

FIG. 7: A graph formed by plotting the evaluation results of the cycle life and discharge capacity using B/A and rA (Å) as coordinate axes, with respect to hydrogen storage alloys of Examples 7 to 42.

FIG. 8: A graph showing the relation of the capacity retention ratio to CaCu₅ phase production ratio, with respect to hydrogen storage alloys of Examples 82 to 91.

FIG. 9: A graph showing the relation of the remaining discharge capacity to hydrogen equilibrium pressure, with respect to hydrogen storage alloys of Examples 92 to 101.

FIG. 10: A graph formed by plotting the Ce content and the cycle life, with respect to hydrogen storage alloys of Examples 102 to 109.

BEST MODES OF THE EMBODIMENTS

(First Hydrogen Storage Alloy)

The first hydrogen storage alloy of the invention contains two or more crystal phases having crystal structures different from one another and layered in the c-axis direction of the crystal structures.

Examples of the above-mentioned crystal phases are a crystal phase having a rhombohedral La₅MgNi₂₄ type crystal structure (hereinafter, sometimes referred to simply La₅MgNi₂₄ phase), a crystal phase having a hexagonal Pr₅Co₁₉ type crystal structure (hereinafter, sometimes referred to simply Pr₅Co₁₉ phase), a crystal phase having a rhombohedral Ce₅Co₁₉ type crystal structure (hereinafter, sometimes referred to simply Ce₅Co₁₉ phase), a crystal phase having a hexagonal Ce₂Ni₇ type crystal structure (hereinafter, sometimes referred to simply Ce₂Ni₇ phase), a crystal phase having a rhombohedral Gd₂Co₇ type crystal structure (hereinafter, sometimes referred to simply Gd₂Co₇ phase), a crystal phase having a hexagonal CaCu₅ type crystal structure (hereinafter, sometimes referred to simply CaCu₅ phase), a crystal phase having a cubic AuBe₅ type crystal structure (hereinafter, sometimes referred to simply AuBe₅ type phase), and a crystal phase having a rhombohedral PuNi₃ type crystal structure (hereinafter, sometimes referred to simply PuNi₃ phase).

Particularly, it is preferable to layer two or more kind crystal phases selected from a group consisting of La₅MgNi₂₄ phase, Pr₅Co₁₉ phase, Ce₅Co₁₉ phase, and Ce₂Ni₇ phase. A hydrogen storage alloy having layered structure of these crystal phases cause less strains since the difference of the expansion and shrinkage ratio among the respective crystal phases and thus has an excellent property that less deterioration is caused at the time of repeating hydrogen absorption and release.

Herein, the La₅MgNi₂₄ type crystal structure means a crystal structure formed by inserting 4 AB₅ units between A₂B₄ units: the Pr₅Co₁₉ type crystal structure means a crystal structure formed by inserting 3 AB₅ units between A₂B₄ units: the Ce₅Co₁₉ type crystal structure means a crystal structure formed by inserting 3 AB₅ units between A₂B₄ units: the Ce₂Ni₇ type crystal structure means a crystal structure formed by inserting 2 AB₅ units between A₂B₄ units: the Gd₂Co₇ type crystal structure means a crystal structure formed by inserting 2 AB₅ units between A₂B₄ units: and the AuBe₅ type crystal structure means a crystal structure composed of A₂B₄ units alone.

In addition, the A₂B₄ unit means a crystal lattice having a hexagonal MgZn₂ type crystal structure (C14 structure) or a hexagonal MgCu₂ type crystal structure (C15 structure) and AB₅ unit means a crystal lattice having a hexagonal CaCu₅ type crystal structure.

Further, A denotes any element selected from a group consisting of rare earth metal elements and Mg and B denotes any element selected from a group consisting of transition metal elements and Al.

The layering order of the above-mentioned respective crystal phases is not particularly limited and any specified crystal phase combination may be layered repeatedly with periodicity or the respective crystal phases may be layered at random without periodicity.

In addition, with respect to the crystal phases having the above-mentioned respective crystal structures, the crystal structures can be specified by carrying out X-ray diffraction for milled alloy powders and analyzing the obtained X-ray diffraction patterns by Rietveld method.

A schematic drawing of one embodiment of the first hydrogen storage alloy is shown in FIG. 1. As shown in FIG. 1, one embodiment of the first hydrogen storage alloy is configured by layering the CaCu₅ phase, two Pr₅Co₁₉ phases neighboring to the CaCu₅ phase, and two Ce₂Ni₇ phase neighboring to the Pr₅Co₁₉ phases in the c-axis direction of the crystal structure.

Observation of the lattice image of the alloy by TEM makes it possible to confirm the fact that two or more crystal phases having different crystal structures are layered in the c-axis direction of the crystal structures. One example of the lattice image of the first hydrogen storage alloy of the invention is shown in FIG. 2 and FIG. 3.

These drawings shows that this hydrogen storage alloy is configured by layering a crystal phase formed by repeating arrangement of 3 AB₅ units inserted between A₂B₄ units and a crystal phase formed by repeating arrangement of 4 AB₅ units inserted between A₂B₄ units in the c-axis direction. The former crystal phase is a crystal phase having the Ce₅Co₁₉ phase crystal structure and the latter is a crystal phase having the LaMgNi₂₄ type crystal structure. Observation of the lattice image by TEM in such a manner makes it possible to confirm the fact that two or more crystal phases having different crystal structures are layered in the c-axis direction, which is a constituent factor of the invention.

As described, since the first hydrogen storage alloy of the invention comprises two or more crystal phases having different crystal structures and layered in the c-axis direction, the strains of the crystal phases at the time of absorbing hydrogen can be moderated by neighboring other crystal phases. Accordingly, even if hydrogen absorption and release are repeated, less pulverization of the alloy is caused and thus an excellent cycle life can be caused.

(Second Hydrogen Storage Alloy)

The second hydrogen storage alloy of the invention is configured by adjusting the difference of the maximum value and the minimum value of the lattice constant a (hereinafter, also referred to as a-axis length) in the crystal structures of the layered two or more crystal phases to be 0.03 Å or less in the first hydrogen storage alloy.

When the difference of the maximum value and the minimum value of the a-axis length of the respective crystal phases is configured by adjusted to be 0.03 Å or less, the strains among the respective crystal phases caused at the time of hydrogen absorption and release are further reduced and the hydrogen storage alloy becomes difficult to be pulverized even if hydrogen absorption and release are repeated, that is, the hydrogen storage alloy becomes excellent in the cycle life.

The difference of the maximum value and the minimum value of the a-axis length of the respective crystal phases is preferably 0.02 Å or less, more preferably 0.016 Å or less, and even more preferably 0.01 Å or less.

When the difference of the maximum value and the minimum value of the a-axis length is in the above-mentioned range, the capacity retention ratio of the hydrogen storage alloy is further improved and the cycle life can be improved.

Herein, the a-axis length in the invention can be measured by carrying out crystal structure analysis of the hydrogen storage alloy by an X-ray diffraction apparatus. More practically, the a-axis length can be calculated for each crystal phase by determining the lattice constant of each crystal phase from XRD patterns by Rietveld method (analysis software: RIETAN2000).

The above-mentioned first or second hydrogen storage alloy is preferable to have a composition defined by a general formula R1_(a)R2_(b)R3_(c) (wherein R1 is one or more kind elements selected from a group consisting of rare earth metals including Y; R2 is one or more kind elements selected from a group consisting of Mg, Ca, Sr, and Ba; R3 is one or more kind elements selected from a group consisting of Ni, Co, Mn, Al, Cr, Fe, Cu, Zn, Si, Sn, V, Nb, Ta, Ti, Zr, and Hf; and a, b, and c satisfy 10≦a≦30; 1≦b≦10; 65≦c≦90; and a+b+c=100).

(Third Hydrogen Storage Alloy)

The third hydrogen storage alloy of the invention is the first hydrogen storage alloy further having a composition defined by a general formula R1_(d)R2_(e)R4_(f)R5_(g) (wherein R1 is one or more kind elements selected from a group consisting of rare earth metals including Y; R2 is one or more kind elements selected from a group consisting of Mg, Ca, Sr, and Ba; R4 is one or more kind elements selected from a group consisting of Ni, Co, Cr, Fe, Cu, Zn, Si, Sn, V, Nb, Ta, Ti, Zr, and Hf; R5 is one or two elements selected from Mn and Al; and d, e, f, and g satisfy 8≦d≦19; 2≦e≦9; 73≦f≦79; 1≦g≦4; and d+e+f+g=100) and satisfying 3.53≦(B/A)≦3.80 and 0.0593(B/A)+1.59≦rA≦0.0063(B/A)+1.81, preferably 0.0593(B/A)+1.59≦rA≦1.827, in the case (B/A) is defined as (f+g)/(d+e) and rA (Å) is defined as the average atomic radius of R1 and R2.

In the case the average atom radius rA (Å) of the R1 and R2 elements (that is, the element in the A side) composing the crystal structures of the hydrogen storage alloy and the ratio (B/A) of the R1 and R2 elements (that is, the elements in the A side) to the R4 and R5 elements (that is, the elements in the B side) satisfy the following relational expressions: 3.53≦(B/A)≦3.80 and 0.0593(B/A)+1.59≦rA≦0.0063(B/A)+1.81: the R2 element tends to be included in the A₂B₄ units and as a result, segregation of the R2 element is prevented and it becomes easy to form the layered body of the crystal phases having desired crystal structures and accordingly, a hydrogen storage alloy excellent in the cycle life can be obtained.

The third hydrogen storage alloy, preferably, wherein the R1 consists of one or more kind elements R1′ selected from a group consisting of Ce, Pr, Nd, Sm, and Y and La at La/R1′ ratio or 5 or less; the R2 is Mg; R4 is one or two elements selected from Ni and Co; R5 is Al.

In the case where La is substituted with one or more kind elements R1′ selected from a group consisting of Ce, Pr, Nd, Sm, and Y whose atomic radius are smaller than the La at La/R1′ ratio or 5 or less, and the R2 is Mg, the R4 is one or two elements selected from Ni and Co, the R5 is Al, and the d, e, f, and g respectively satisfy 16≦d≦19, 2≦e≦5, 73≦f≦78, and 2≦g≦4; Mg as the R2 element tends to be included further easier in the A₂B₄ units and accordingly, a hydrogen storage alloy excellent in the cycle life can be obtained.

Third hydrogen storage alloy preferably contains a crystal phase having Pr₅Co₁₉ type crystal structure or a crystal phase having Ce₅Co₁₉ type crystal structure and further preferably contains the crystal phase as the main produced phase.

When the crystal phase having Pr₅Co₁₉ type crystal structure or the crystal phase having Ce₅Co₁₉ type crystal structure is the main produced phase, the lattice expansion coefficient is small at the time of hydrogen absorption and resulted in an action that less strains are caused, thereby giving an effect to further improve the cycle life.

Herein, the main produced phase means the phase at the highest production ratio.

(Fourth Hydrogen Storage Alloy)

The fourth hydrogen storage alloy of the invention is configured to have, in the said first hydrogen storage alloy, a crystal phase having Ce₅Co₁₉ type crystal structure and a composition defined by a general formula La_(h)R6_(i)R7_(j)Mg_(k)R8_(m) (wherein R6 is one or more kind elements selected from a group consisting of rare earth metals including Y and excluding La; R7 is one or more kind elements selected from a group consisting of Zr, Ti, Zn, Sn and V; R8 is one or more kind elements selected from a group consisting of Ni, Co, Mn, Al, Cu, Fe, Cr, and Si; and h, i, j, k and m satisfy 0.70≦h/(h+i)≦0.85; and h+i+j+k+m=100).

According to the fourth hydrogen storage alloy, since the crystal phase having Ce₅Co₁₉ type crystal structure is contained as an indispensable phase, the alloy becomes excellent in the cycle life and furthermore, since the ratio h/(h+i) of La to the total of La and R6 element is in a range satisfying 0.70≦h/(h+i)≦0.85, segregation of Mg can be prevented and the production ratio of the crystal phase having the CaCu₅ type crystal structure which is inferior in the cycle life can be decreased and on the other hand, the ratio of the crystal phase having the Ce₅Co₁₉ type crystal structure which is excellent in the cycle life is increased and as a result, a hydrogen storage alloy having high hydrogen storage capacity and excellent in the cycle life can be obtained.

In the fourth hydrogen storage alloy, j is preferable to be 0 or higher and 0.65 or lower and more preferable to be 0.2 or higher and 0.65 or lower. When j is in the above-mentioned numeral range, because of the existence of the R7 element (that is, one or more kind elements selected from a group consisting of Zr, Ti, Zn, Sn and V), Mg becomes diff cult to segregate and the ratio of the crystal phase having the Ce₅Co₁₉ type crystal structure is increased and the hydrogen storage capacity is increased.

Further, in the fourth hydrogen storage alloy, k is preferable to be 2 or higher and 5.5 or lower and more preferable to be 3 or higher and 5 or lower. When k is in the above-mentioned numeral range, segregation of Mg is prevented and the hydrogen storage alloy is provided with high hydrogen storage capacity and becomes excellent in the cycle life.

(Fifth Hydrogen Storage Alloy)

The fifth hydrogen storage alloy of the invention is the hydrogen storage alloy which contains 22% by weight or less of the crystal phase having the CaCu₅ type crystal structure according to the first hydrogen storage alloy.

Conventionally, it is known that the crystal phase having the CaCu₅ type crystal structure is excellent in the cycle life despite of a low discharge capacity. However, according to the results of the investigations which the inventors of the invention have made, it is found that in a hydrogen storage alloy configured by layering two or more layers of crystal phases having crystal structures different from one another, if the CaCu₅ phase exists much, the cycle life contrarily becomes difficult to be improved.

The fifth hydrogen storage alloy is made further excellent in the cycle life by adjusting the CaCu₅ phase ratio to be 22% by weight or less.

Further, in the fifth hydrogen storage alloy, the hydrogen equilibrium pressure is 0.07 MPa or lower.

Conventionally, a hydrogen storage alloy has a property that hydrogen absorption is difficult and release of absorbed hydrogen is easy in the case of high hydrogen equilibrium pressure. Accordingly, if the high rate capability is improved for the hydrogen storage alloy, self-release of hydrogen becomes easy.

However, as a result of the investigations by the inventors, it is found that a good high rate capability can be obtained in the case where the hydrogen equilibrium pressure is set to be as low as 0.07 MPa or lower in the hydrogen storage alloy comprising two or more layers of crystal phases having crystal structures different from one another and containing 22% by weight or less of the crystal phase having the CaCu₅ type crystal structure. This seems due to improvement in diffusivity of hydrogen in the alloy.

Accordingly, in the fifth hydrogen storage alloy, the high rate capability is made excellent and self-release of hydrogen (in the case of a battery, self discharge) becomes difficult by setting the hydrogen equilibrium pressure to be 0.07 MPa or lower.

Herein, the hydrogen equilibrium pressure means the equilibrium pressure at H/M=0.5 (equilibrium pressure in the case the ratio of hydrogen atoms to metal atoms is 0.5) in the PCT curve (pressure-composition isothermal line) at 80° C.

With respect to the fifth hydrogen storage alloy, the Mn content in the alloy is preferable to be 5% by weight or less. When the Mn content is adjusted to be 5% by weight or less, while the high rate capability is maintained, the self-release of hydrogen can further be suppressed.

Further, the fifth hydrogen storage alloy is preferable to have a composition defined by a general formula R1_(n)R2_(p)R4_(q)R5_(r) (wherein R1 is one or more kind elements selected from a group consisting of rare earth metals including Y; R2 is one or more kind elements selected from a group consisting of Mg, Ca, Sr, and Ba; R4 is one or more kind elements selected from a group consisting of Ni, Co, Cr, Fe, Cu, Zn, Si, Sn, V, Nb, Ta, Ti, Zr, and Hf; R5 is one or two kind elements selected from Mn and Al; and n, p, q, and r satisfy 16≦n≦23; 2≦p≦8; 68.5≦q≦76; 1≦r≦6.5; and n+p+q+r=100).

In terms of saving material cost, the hydrogen storage alloy according to the invention, misch metals (including La, Ce, Nd, and Pr) are preferable to be used as raw materials. Use of the misch metals as raw materials, the use amounts of costly high purity materials such as neodymium and praseodymium can be suppressed and at the same time same effect as that in the case of using the high purity materials can be caused.

In the case the misch metals are used as raw materials, the cerium content in the hydrogen storage alloy of the invention is preferable to be 2.2 mol % or less. When the cerium content is adjusted to be 2.2 mol % or less, decrease of the cycle life can be suppressed. The effect of suppressing the decrease of the cycle life become significant when the cerium content is 1.3 mol % or less. Particularly, when the cerium content is adjusted to be 0.9 mol % or less, the decrease of the cycle life can be suppressed to an extremely low level.

Further, in the case where the above-mentioned misch metals are used as raw materials, in the hydrogen storage alloy of the invention, it is preferable that the total ratio of the Pr₅Co₁₉ phase, Ce₅Co₁₉ phase, and Ce₂Ni₇ phase is 95% by weight or higher. With such configuration, an excellent cycle life is exhibited. Particularly, in the case where the total ratio of these three phases is 98% by weight or higher, the effect becomes furthermore significant. Such an effect is supposedly attributed to the suppression of pulverization owing to the uniform alloy structure.

Next, a method for producing a hydrogen storage alloy of the invention will be described.

A method for producing the first hydrogen storage alloy involves a melting step of melting alloy raw materials mixed at a prescribed composition ratio, a cooling step of rapid solidification the molten alloy raw materials at a cooling speed of 1000 K/s or higher, and an annealing step of annealing the cooled alloy at temperature range of 860° C. or higher and 1000° C. or lower in inert gas atmosphere in pressurized state.

Herein, in the case the composition ratio of the alloy raw materials is defined by a general formula R1_(a)R2_(b)R3_(c) (wherein R1 is one or more kind elements selected from a group consisting of rare earth metals including Y; R2 is one or more kind elements selected from a group consisting of Mg, Ca, Sr, and Ba; R3 is one or more kind elements selected from a group consisting of Ni, Co, Mn, Al, Cr, Fe, Cu, Zn, Si, Sn, V, Nb, Ta, Ti, Zr, and Hf), s, b, and c satisfy 8≦s≦30; 1≦b≦10; 65≦c≦90; and s+b+c=100.

If the hydrogen storage alloy is produced by such a production method, it is made possible to obtain the hydrogen storage alloy having two or more layered crystal phases having crystal structures different from one another.

To explain it more concretely, at first, based on the chemical composition of the aimed hydrogen storage alloy, prescribed amount of a raw material ingot (an alloy raw material) is weighed.

In the melting step, the above-mentioned alloy raw material is put in a crucible and heated at, for example, 1200° C. or higher and 1600° C. or lower, to melt the alloy raw material using a high frequency melting furnace in an inert gas atmosphere or vacuum.

In the cooling step, the melted alloy raw material is cooled and solidified. The cooling speed is preferably 1000 K/s or higher (also referred to as quenching). Quenching at 1000 K/s or higher is effective to make the alloy structure very fine and uniform. Further, the cooling speed can be set in a range of 1,000,000 K/s or lower.

As the cooling method, practically, a melt spinning method at a cooling speed of 100,000 K/s or higher and a gas atomization method at a cooling speed of about 10,000 K/s or higher can preferably be employed.

In the annealing step, in the pressurized state in inert gas atmosphere, heating at 860° C. or higher and 1000° C. or lower may be carried out using, for example, an electric furnace or the like. As the pressurizing condition, it is preferable to be 0.2 MPa (gauge pressure) or higher and 1.0 MPa (gauge pressure) or lower. Further, the treatment time for the annealing step is preferably 3 hours or longer and 50 hours or shorter.

Such an annealing step is effective to release strains of crystal lattice and the hydrogen storage alloy subjected to the annealing step finally becomes the hydrogen storage alloy comprising two or more layered crystal phases having crystal structures different from one another.

A method for producing the second hydrogen storage alloy is the method for producing according to the first hydrogen storage alloy in which the temperature condition of the annealing step is adjusted to be 890° C. or higher and 970° C. or lower.

Adjustment of the condition in such a manner makes diffusion of atoms relatively easy, suppresses evaporation of Mg, Ca, Sr, and Ba, easily uniformalizes the length of the a-axis in the respective crystal phases, and gives the hydrogen storage alloy having the difference of 0.03 Å between the maximum value and the minimum value.

In the method for producing the second hydrogen storage alloy, the temperature condition in the annealing step is preferable to be 900° C. or higher and 940° C. or lower. Adjustment of the condition in such a manner is effective to make the concentration distribution of respective constituent elements uniform and make the difference of the a-axis length among the produced phases further shorter.

A method for producing the third hydrogen storage alloy is the method for producing according to the first hydrogen storage alloy in which the ratio of R1 and R2 elements of the alloy raw material and the ratio of R4 and R5 elements are adjusted and the above-mentioned average atom radium rA (Å) and the ratio (B/A) are satisfied in the case of consideration of the atom radius of the respective elements.

Further, in the method for producing the third hydrogen storage alloy, the average atom radium rA (Å) and the ratio (B/A) are preferable to satisfy 1.810≦rA≦1.825 and 3.60≦(B/A)≦3.70.

Adjustment of the condition in such a manner is effective to maintain the high capacity and at the same time to give further improved cycle performance.

A method for producing the fourth hydrogen storage alloy is the method for producing the first hydrogen storage alloy in which the alloy raw material composition is adjusted to give the composition of the fourth hydrogen storage alloy and the temperature condition in the annealing step is adjusted to be 890° C. or higher and 970° C. or lower.

Adjustment of the condition in such a manner makes diffusion of atom relatively easy and effective to suppress evaporation of Mg, Ca, Sr, and Ba, satisfy the prescribed composition, and obtain the hydrogen storage alloy containing the crystal phase having the Ce₅Co₁₉ type crystal structure.

Further, in the method for producing the fourth hydrogen storage alloy, it is more preferable that in the above-mentioned composition La_(h)R6_(i)R7_(j)Mg_(k)R8_(m), k satisfies 3.4<k<4.3; R7 consists of one or more kind elements R7′ selected from a group consisting of Zr, Zn, and Sn in combination with Ti and is defined as R7=Ti_(t)R7′_(j-t) (wherein 0≦t<0.3); and R8 consists of one or more kind elements R8′ selected from a group consisting of Ni, Co, Cu, Fe, and Cr in combination with Mn and defined as R8=Mn_(a)R8′_(m-s) (wherein 0<s<1.1).

Adjustment of the condition in such a manner is effective to maintain the high capacity and at the same time to further improve the cycle performance.

The method for producing the fifth hydrogen storage alloy is the method for producing according to the first hydrogen storage alloy in which the temperature condition in the annealing step is adjusted to be 890° C. or higher and 970° C. or lower.

Adjustment of the condition in such a manner makes diffusion of atom relatively easy and effective to suppress evaporation of Mg, Ca, Sr, and Ba, satisfy the prescribed composition, and obtain the hydrogen storage alloy containing 20% by weight or less of the crystal phase having the CaCu₅ type crystal structure.

In the method for producing the fifth hydrogen storage alloy, it is more preferable that in the composition La_(u)R9_(v)Mg_(w)R10_(x)R11_(y) (wherein, R9 is at least one element of Pr and Nd; R10 is at least one element of Ni and Co; R11 is at least one element of Al and Mn; and u, v, w, x, and y satisfy 4.255≦u≦17.39; 0≦v≦13.62; 2.128≦w≦4.701; 72.30≦x≦77.66; and 1.06≦y≦6.38).

Adjustment of the condition in such a manner is effective to make Mg, Mn, and Al occupy specified atom sites in the layered structure, stabilize their structure, and suppress production of the crystal phase having the CaCu₅ type crystal structure.

A hydrogen storage alloy electrode of the invention is provided with the above-mentioned hydrogen storage alloy as a hydrogen storage medium. At the time of using the hydrogen storage alloy of the invention for an electrode as a heat storage medium, it is preferable to pulverize the hydrogen storage alloy for the use.

The pulverization of the hydrogen storage alloy at the time of electrode production may be carried out either before or after annealing, however, since the surface area becomes wide by the pulverization, in terms of prevention of surface oxidation of the alloy, it is desirable to pulverize the alloy after annealing. The pulverization is preferable to carry out in inert atmosphere for oxidation prevention of the alloy surface.

The pulverization may be carried out by, for example, mechanical pulverization, hydrogenation pulverization, and the like.

Further, a secondary battery of the invention is configured to be a nickel-metal hydride battery using the hydrogen storage alloy as a negative electrode. Since the hydrogen storage alloy of the invention, that is the hydrogen storage alloy electrode, has corrosion resistance to an aqueous strongly alkaline solution to be used as an electrolytic solution of a nickel-metal hydride battery or the like, it is excellent in the cycle performance in the case where hydrogen absorption and release are repeatedly carried out. As a result, the charge and discharge cycle performance of the secondary battery also become excellent.

In addition, as a positive electrode of the secondary battery, for example, nickel electrode (sintered type or non-sintered type) is employed.

EXAMPLES

Hereinafter, the invention will be described more practically, referring to Examples and Comparative Examples; however the invention should not be limited to the following Examples.

Example 1

A prescribed amount of a raw material ingot having the chemical composition shown in Table 1 was weighed, put in a crucible, and heated at 1500° C. in reduced pressure argon atmosphere using a high frequency melting furnace to melt the material. After the melting, the melted alloy was quenched by employing a melt spinning method and solidified.

Next, the obtained alloy was heated at 910° C. in 0.2 MPa (gauge pressure, hereinafter the same) of pressurized argon gas.

Comparative Example 1

A prescribed amount of a raw material ingot having the chemical composition shown in Table 1 was weighed, put in a crucible, and heated at 1500° C. in reduced pressure argon atmosphere using a high frequency melting furnace to melt the material. After the melting, the melted alloy was quenched by employing a melt spinning method and solidified.

Next, the obtained alloy was heated at 910° C. in 0.2 MPa (gauge pressure, hereinafter the same) of pressurized argon gas.

TABLE 1 La Pr Mg Ni Co Al Example 1 13.3 4.2 3.3 71.9 4.2 3.1 Comparative 12.8 4.3 4.3 68.1 6.4 4.3 Example 1 <Measurement of Crystal Structure and Calculation of Existence Ratio>

Each obtained hydrogen storage alloy was pulverized to obtain a powder with an average particle diameter (D50) of 20 μm and the powder was subjected to measurement under condition of 40 kV and 100 mA (Cu bulb) using an X-ray diffraction apparatus (manufactured by Bruker AXS: model number M06XCE). Further, as structure analysis, analysis by Rietveld method (analysis software: RIETAN 2000) was carried and the production ratios of the produced crystal phases in each hydrogen storage alloy were calculated.

The ratios (% by weight) of the produced phases are shown in Table 2.

TABLE 2 Ce₅Co₁₉ Pr₅Co₁₉ phase phase CaCu₅ phase AuBe₅ phase Total Example 1 62.7 28.5 8.74 0.0 100 Comparative 69.1 16.1 11.8 3.0 100 example 1 <Evaluation of Distribution State of Crystal Phase>

With respect to the obtained hydrogen storage alloy powders of Example and Comparative Example, the distribution state (color map) of Ni and Mg was observed using EPMA (Electron Probe Micro Analyzer) to evaluate the distribution state of produced phases. With respect to Example 1 and Comparative Example 1, FIG. 4 shows photographs of the distribution state (color map) of Ni and Mg obtained by EPMA.

As shown in FIG. 4, it is observed that both Ni and Mg were distributed uniformly in the hydrogen storage alloy of Example and according to Rietveld analysis result or the like for the alloy, it was confirmed that a plurality of phases were produced and as comprehensive analysis of these results, it could be confirmed that the crystal phases existed in layered state in the hydrogen storage alloy. On the other hand, in the hydrogen storage alloy of Comparative Example, Ni and Mg were distributed ununiformly in places and thus it was supposed that the crystal phases separately existed without forming a layered body. In addition, when the lattice image of the hydrogen storage alloy of Example 1 was observed by transmission electron microscope (TEM), the crystal phases with the crystal structures different from one another were layered in the c-axis direction.

<Evaluation of Cycle Performance>

(a) Production of Electrode

After 3 parts by weight of a nickel powder (manufactured by INCO, #210) was added to 100 parts by weight of each of the obtained hydrogen storage alloy powders of Example and Comparative Example, an aqueous solution in which a thickener (methyl cellulose) was dissolved was added and further 1.5 parts by weight of a binder (styrene-butadiene rubber) was added to obtain a paste which was applied to both faces of a punched steel plate (porosity 60%) with a thickness of 45 μm and dried and thereafter pressed in 0.36 mm thickness to obtain a negative electrode. On the other hand, a sintered type nickel hydroxide electrode with an excess capacity was used as a positive electrode.

(b) Production of Flooded Cell

The negative electrode produced in the above-mentioned manner was sandwiched with positive electrodes while inserting separators between them and these electrodes were fixed with bolts at pressure of 1 kgf/cm² to assemble an opened type cell. A mixed solution of a 6.8 mol/L KOH solution and a 0.8 mol/L LiOH solution was used as an electrolyte solution.

(c) Measurement Method of Discharge Capacity

In a water bath at 20° C., charging and discharging was repeated 65 cycles in condition of charging at 0.1 ItA to 150%, discharging at 0.2 ItA to cut off voltage of −0.6 V (vs. Hg/HgO). The results are shown in FIG. 5.

As shown in FIG. 5, with respect to the hydrogen storage alloy of Comparative Example 1 in which the crystal phases existed separately, the discharge capacity was lowered to about 88% after the 65 cycles, whereas with respect to the hydrogen storage alloy of Example 1, it was confirmed that the discharge capacity was maintained at 99.7% even after 65 cycles.

Examples 2 to 6

Using the alloy materials with the compositions shown in the following Table 3, hydrogen storage alloys of Examples 2 to 6 were produced in the same manner as Example 1. When the lattice images of these hydrogen storage alloys were observed by a transmission electron microscope (TEM), it was confirmed that crystal phases with different crystal structures were layered in the c-axis direction.

TABLE 3 Unit: mol % La Pr Mg Ni Co Mn Al Example 2 12.58 4.19 4.19 71.28 2.10 2.10 2.10 Example 3 17.19 0.00 3.77 68.13 6.29 0.00 3.14 Example 4 9.64 8.18 3.14 75.89 0.00 0.00 1.68 Example 5 4.30 14.46 2.09 77.50 1.65 0.00 0.00 Example 6 5.01 14.08 2.12 74.50 4.29 0.00 0.00 <Measurement of a-Axis Length>

With respect to each of the obtained hydrogen storage alloys, the production ratios of the crystal phases were calculated and at the same time, the XRD patterns were measured by an X-ray diffraction apparatus and the a-axis length for each produced crystal phase was calculated by Rietveld method (analysis software: RIETAN 2000). The results are shown in the following Table 4 and FIG. 6.

Further, in the same manner as Example 1, the ratios of the produced phases of each hydrogen storage alloy and the retention ratio of the discharge capacity after 50 cycles were measured. The results are also shown in Table 4.

TABLE 4 A-axis Capacity Ratios of produced crystal phase (% by weight) length retention [A-axis length (Å)] difference ratio Ce₂Ni₇ phase Ce₅Co₁₉ phase Pr₅Co₁₉ phase CaCu₅ phase (Å) (%) Example 2 36.95 38.26 14.04 10.75 0.003 93.42 [5.045] [5.044] [5.044] [5.043] Example 3 26.43 35.11 20.42 18.04 0.016 92.87 [5.066] [5.088] [5.059] [5.050] Example 4 42.32 37.89 13.13 6.67 0.036 90.25 [5.028] [5.017] [5.020] [4.992] Example 5 32.9 40.0 12.6 14.5 0.033 90.3 [5.025] [5.015] [5.022] [4.992] Example 6 51.3 30.6 8.50 9.60 0.030 93.4 [5.033] [5.029] [5.034] [5.004]

Examples 7 to 42

Using the alloy raw materials of the compositions shown in the following Table 5, hydrogen storage alloys of Examples 7 to 42 were produced in the same manner as Example 1. When the lattice images of these hydrogen storage alloys were observed by a transmission electron microscope (TEM), it was confirmed that crystal phases with different crystal structures were layered in the c-axis direction.

Comparative Example 2

Similarly, using the allow raw material shown in Table 5, a hydrogen storage alloy of Comparative Example 2 was produced in the same manner as Comparative Example 1.

TABLE 5 Capacity Main retention Raw material composition produced rA Produced phase (%) Capacity ratio La Ce Pr Nd Sm Y Mg Ni Co Mn Al B/A phase Å Ce2Ni7 Pr5Co19 Ce5Co19 La4MgNi24 CaCu5 Others mAh/g % Evaluation Example 7 12.4 0.0 4.1 0.0 0.0 0.0 4.1 71.1 4.1 1.0 3.1 3.85 Ce2Ni7 1.8122 43 6 32 5 14 351 89.3 Example 8 12.8 0.0 4.3 0.0 0.0 0.0 4.3 70.2 4.3 1.1 3.2 3.70 Pr5Co19 1.8122 18 34 21 21 6 352 94.5 ● Example 9 12.9 0.0 4.3 0.0 0.0 0.0 4.3 69.9 4.3 1.1 3.2 3.65 Pr5Co19 1.8122 28 50 12 6 4 349 96 ● Example 10 13.2 0.0 4.4 0.0 0.0 0.0 4.4 69.1 4.4 1.1 3.3 3.53 Pr5Co19 1.8122 40 42 10 4 4 347 95.4 ● Example 11 13.4 0.0 4.5 0.0 0.0 0.0 4.5 68.8 4.5 1.1 3.3 3.48 Ce2Ni7 1.8122 50 30 9 11 340 91.6 ◯ Example 12 13.8 0.0 3.1 0.0 0.0 0.0 3.7 71.1 4.1 1.0 3.1 3.85 Ce5Co19 1.8202 32 17 44 7 354 92.9 ◯ Example 13 16.5 0.0 2.1 0.0 0.0 0.0 2.1 71.1 4.1 1.0 3.1 3.85 Pr5Co19 1.8446 55 33 12 325 88.6 Δ Example 14 17.2 0.0 0.0 0.0 0.0 0.0 4.3 71.0 4.3 0.0 3.2 3.65 Ce5Co19 1.822 20 28 42 10 355 91.3 ◯ Example 15 14.1 0.0 3.7 0.0 0.0 0.0 3.9 71.7 4.3 0.0 2.2 3.60 Pr5Co19 1.8192 20 42 34 4 350 97 ● Example 16 13.0 0.0 0.0 4.3 0.0 0.0 4.3 72.8 2.2 0.0 3.3 3.60 Pr5Co19 1.8108 10 45 40 5 348 97.5 ● Example 17 13.0 0.0 0.0 5.2 0.0 0.0 3.5 72.8 2.2 0.0 3.3 3.60 Pr5Co19 1.8196 26 60 14 347 97.8 ● Example 18 9.8 0.0 6.5 1.1 0.0 0.0 4.3 71.7 4.3 0.0 2.2 3.60 Pr5Co19 1.8045 36 40 20 4 350 96.5 ● Example 19 6.5 0.0 10.9 0.0 0.0 0.0 4.3 71.7 4.3 0.0 2.2 3.60 Ce2Ni7 1.7975 45 18 30 7 324 94.6 ◯ Example 20 12.8 0.0 0.0 3.2 0.0 0.0 5.3 72.3 4.3 0.0 2.1 3.70 PuNi3 1.7999 15 19 32 34 321 90.6 Δ Example 21 12.8 0.0 0.0 5.3 0.0 0.0 3.2 72.3 4.3 0.0 2.1 3.70 Pr5Co19 1.8218 58 36 6 354 97.5 ● Example 22 17.9 0.0 0.0 1.1 0.0 0.0 2.3 72.3 4.3 0.0 2.1 3.70 Pr5Co19 1.844 24 55 17 4 337 92.6 Δ Example 23 3.9 8.7 1.1 4.3 0.2 0.0 3.5 71.7 4.3 0.0 2.2 3.60 Ce2Ni7 1.7978 39 19 32 10 325 88 Δ Example 24 14.0 0.0 0.0 2.1 0.0 2.1 3.0 73.4 2.1 1.1 2.1 3.70 Ce5Co19 1.8253 30 28 31 6 5 347 96 ● Example 25 12.5 0.0 0.0 0.0 0.0 4.2 4.2 69.8 6.3 0.0 3.1 3.80 Ce5Co19 1.8068 22 15 40 10 13 360 92.7 ◯ Example 26 12.9 0.0 0.0 0.0 0.0 4.6 3.3 69.8 6.3 0.0 3.1 3.80 Ce5Co19 1.8163 20 19 34 7 20 355 94.9 ● Example 27 14.8 0.2 0.0 0.0 0.0 2.9 2.9 69.8 6.3 0.0 3.1 3.80 Ce5Co19 1.8273 18 22 31 5 6 18 350 94.1 ● Example 28 15.8 0.0 0.0 0.0 0.0 2.5 2.5 69.8 6.3 0.0 3.1 3.80 Ce2Ni7 1.8349 35 25 30 10 331 90.4 Δ Example 29 14.8 2.1 0.0 0.0 0.0 0.0 3.7 73.2 4.1 0.0 2.1 3.85 Ce5Co19 1.8223 16 20 48 16 358 91.5 ◯ Example 30 14.5 2.0 0.0 0.0 0.0 0.0 3.6 73.7 4.0 0.0 2.0 3.95 Ce5Co19 1.8223 20 58 22 352 91.3 ◯ Example 31 16.1 0.0 2.2 0.0 0.0 0.0 4.0 69.9 4.5 0.0 3.3 3.48 Ce2Ni7 1.8226 55 14 21 10 326 88.6 Δ Example 32 11.0 0.0 0.0 6.6 0.0 0.0 4.4 69.1 5.5 0.0 3.3 3.53 Ce5Co19 1.8052 31 23 39 7 341 94.1 ● Example 33 11.2 0.0 0.0 6.7 0.0 0.0 4.5 69.9 4.5 0.0 3.3 3.48 Ce5Co19 1.8052 37 11 41 11 331 92.1 Δ Example 34 12.8 0.0 4.3 0.0 0.0 0.0 4.3 70.2 4.3 0.0 4.3 3.70 Ce5Co19 1.8122 18 38 37 7 337 93 ◯ Example 35 16.3 0.0 0.0 0.0 0.0 0.0 5.4 71.7 4.3 0.0 2.2 3.60 PuNi3 1.8083 21 13 29 37 341 88.9 ◯ Example 36 13.9 0.0 0.0 4.8 0.0 0.0 3.0 71.4 4.3 0.0 2.6 3.62 Pr5Co19 1.8262 55 38 7 346 94.9 ● Example 37 13.9 0.0 0.0 4.3 0.0 0.0 3.5 71.4 4.3 0.0 2.6 3.62 Pr5Co19 1.8218 7 47 43 3 348 95.2 ● Example 38 14.1 0.0 0.0 4.3 0.0 0.0 3.2 71.4 4.3 0.0 2.6 3.62 Pr5Co19 1.8246 50 45 5 347 95 ● Example 39 15.2 0.0 0.0 3.2 0.0 0.0 3.2 71.4 4.3 0.0 2.6 3.62 Pr5Co19 1.8274 53 44 3 352 95.3 ● Example 40 15.4 0.0 0.0 3.3 0.0 0.0 3.3 71.1 4.4 0.0 2.6 3.56 Pr5Co19 1.8274 8 47 42 3 351 95.3 ● Example 41 16.4 0.0 0.0 2.2 0.0 0.0 3.3 71.1 4.4 0.0 2.6 3.56 Pr5Co19 1.8302 39 26 35 357 90.6 ◯ Example 42 3.8 0.0 7.6 7.6 0.0 0.0 3.3 73.3 0.0 0.0 4.4 3.50 Ce2Ni7 1.8001 92 3 5 337 92.6 Δ Comparative 11.5 3.9 0.2 0.8 0.0 0.0 0.0 67.2 4.9 6.6 4.9 5.10 CaCu5 1.8612 100 307 97.5 ◯ example 2

With respect to the hydrogen storage alloys of Examples 7 to 42 and Comparative Example 2, the production ratios of the crystal phases were calculated and cycle performance measurement was carried out as same as Example 1. The maximum values of the discharge capacity and retention ratios of discharge capacity at 50 cycles are shown in Table 5. Those having the maximum values of the discharge capacity of 340 mAh/g or higher and retention ratio of the discharge capacity at 93% or higher are marked with •: those which satisfy either one were marked with ∘: and those which satisfy neither were marked with Δ. The results are shown together in Table 5.

Further, the calculation results of average atom radius rA (Å) of the above-mentioned R1 element and R2 element (elements at A side) and the ratio (B/A) are shown together in Table 5 and the graph formed by plotting the evaluation results in the B/A-rA (Å) coordinate is shown in FIG. 7.

As shown in FIG. 7, in the case of using the hydrogen storage alloys of Examples which satisfy 3.53≦(B/A)≦3.80 and 0.0593(B/A)+1.59≦rA≦0.0063(B/A)+1.81, that almost all of them exhibited excellent discharge capacity and cycle performance.

Further, from Table 5, in the case rA and B/A satisfy the above-mentioned expressions, respectively, the main produced phase tends to be Pr₅Co₁₉ phase or Ce₅Co₁₉ phase and it was confirmed that excellent discharge capacity and cycle performance were exhibited.

Furthermore, from Table 5, with respect to Examples 15 to 18, 21, 24 to 27, 32, 34, and 35 to 40 which satisfy the following: the R1 is one or more kind elements R1′ selected from a group consisting of Ce, Pr, Nd, Sm, and Y and La at La/R1′ ratio of 5 or less; the R2 is Mg; the R4 is one or two elements selected from Ni and Co; the R5 is Al; and d, e, f, and g respectively satisfy 16≦d≦19; 2≦e≦5; 73≦f≦78; and 2≦g≦4, Pr₅Co₁₉ phase and Ce₅Co₁₉ phase were produced preferentially and the uniformity of the alloys was improved.

Examples 43 to 81

Using the alloy raw materials of the compositions shown in the following Table 6, hydrogen storage alloys of Examples 43 to 81 were produced in the same manner as Example 1. When the lattice images of these hydrogen storage alloys were observed by a transmission electron microscope (TEM), it was confirmed that crystal phases with different crystal structures were layered in the c-axis direction.

With respect to the obtained hydrogen storage alloys, the production ratios of the crystal phases were calculated and cycle performance (the maximum values of the discharge capacity and retention ratios of discharge capacity at 50 cycles) measurement was carried out as same as Example 1. The results are shown together in Table 6. The above-mentioned La/R1′ ratios are also shown in Table 6.

TABLE 6 Capacity Alloy composition (mol %) La/ Produced phases (% by weight) Capacity retention La Ce Pr Nd Y Zr Ti V Sn Zn Mg Ni Co Mn Al Fe Cu Si R1′ AB2 Ce2Ni7 Gd2Co7 Pr5Co19 Ce5Co19 CaCu5 Others Total mAh/g ratio % Example 8.3 8.5 0.2 4.3 68.1 6.4 2.1 2.1 100 0.49 3 41 8 33 15 100 336 94.9 43 Example 11.7 5.1 0.2 4.3 68.1 6.4 2.1 2.1 100 0.70 1 39 10 40 10 100 360 96.4 44 Example 13.6 3.2 0.2 4.3 68.1 6.4 2.1 2.1 100 0.81 41 9 41 9 100 364 96.3 45 Example 14.3 2.6 0.2 4.3 68.1 6.4 2.1 2.1 100 0.85 38 12 37 13 100 368 96.1 46 Example 14.7 2.1 0.2 4.3 68.1 6.4 2.1 2.1 100 0.87 30 14 37 19 100 370 94 47 Example 16.8 0.0 0.2 4.3 68.1 6.4 2.1 2.1 100 1.00 26 15 35 24 100 365 93.1 48 Example 17.9 2.1 0.2 1.1 68.1 6.4 2.1 2.1 100 0.89 46 14 30 10 100 125 99.7 49 Example 16.8 2.1 0.2 2.1 68.1 6.4 2.1 2.1 100 0.89 45 17 29 9 100 252 99.5 50 Example 16.2 2.1 0.2 2.8 68.1 6.4 2.1 2.1 100 0.88 44 14 31 11 100 335 98.1 51 Example 13.6 2.1 0.2 5.3 68.1 6.4 2.1 2.1 100 0.86 20 35 45 100 331 93.6 52 Example 12.6 2.1 0.2 6.4 68.1 6.4 2.1 2.1 100 0.86 2 42 56 100 285 93.5 53 Example 13.4 4.3 0.2 3.4 68.1 6.4 2.1 2.1 100 0.76 30 66 4 100 363 97 54 Example 14.5 3.2 0.2 3.4 68.1 6.4 2.1 2.1 100 0.82 31 64 5 100 368 97.1 55 Example 14.5 3.2 0.2 3.4 68.1 6.4 2.1 2.1 100 0.82 23 21 48 8 100 357 95.1 56 Example 14.5 3.2 0.2 3.4 68.1 6.4 2.1 2.1 100 0.82 24 70 6 100 368 97 57 Example 14.5 3.2 0.2 3.4 68.1 6.4 2.1 2.1 100 0.82 4 4 20 62 10 100 369 95.9 58 Example 14.5 3.4 3.4 68.1 6.4 2.1 2.1 100 0.81 31 66 3 100 372 95 59 Example 14.5 2.8 0.6 3.4 68.1 6.4 2.1 2.1 100 0.84 5 24 52 17 2 100 351 96.9 60 Example 14.5 2.3 1.1 3.4 68.1 6.4 2.1 2.1 100 0.86 10 21 42 23 4 100 336 96.1 61 Example 14.5 3.2 0.2 3.4 68.1 6.4 2.1 2.1 100 0.82 31 64 5 100 361 96.9 62 Example 14.5 3.2 0.2 3.4 68.1 6.4 2.1 2.1 100 0.82 31 64 5 100 358 96.8 63 Example 14.5 3.2 0.2 3.4 68.1 6.4 2.1 2.1 100 0.82 31 64 5 100 361 97 64 Example 16.6 3.7 0.2 3.9 63.4 7.3 2.4 2.4 100 0.82 43 5 31 21 100 335 95.9 65 Example 15.1 3.3 0.2 3.6 66.7 6.7 2.2 2.2 100 0.82 61 18 21 100 342 97.1 66 Example 14.2 3.1 0.2 3.3 68.8 6.3 2.1 2.1 100 0.82 24 20 45 11 100 372 95.7 67 Example 13.3 2.9 0.2 3.1 70.6 5.9 2.0 2.0 100 0.82 3 38 29 30 100 375 95.1 68 Example 14.5 3.2 0.2 3.4 68.1 6.4 4.3 100 0.82 33 24 31 6 6 100 356 97.3 69 Example 14.5 3.2 0.2 3.4 68.1 4.3 2.1 2.1 2.1 100 0.82 32 44 21 3 100 346 95.1 70 Example 14.5 3.2 0.2 3.4 68.1 4.3 2.1 2.1 2.1 100 0.82 30 40 26 4 100 344 95.2 71 Example 14.5 3.2 0.2 3.4 68.1 4.3 2.1 2.1 2.1 100 0.82 28 46 20 6 100 340 96.1 72 Example 14.8 3.3 0.2 3.5 69.6 4.3 1.1 3.3 100 0.82 63 6 24 7 100 364 97.3 73 Example 14.5 3.2 0.2 3.4 70.2 4.3 1.1 3.2 100 0.82 55 4 31 10 100 366 97.1 74 Example 14.2 3.1 0.2 3.3 70.8 4.2 1.0 3.1 100 0.82 44 17 30 9 100 366 96.4 75 Example 13.9 3.1 0.2 3.3 71.4 4.1 1.0 3.1 100 0.82 24 21 42 13 100 361 95.4 76 Example 13.6 3.0 0.2 3.2 72.0 4.0 1.0 3.0 100 0.82 20 28 30 22 100 350 95 77 Example 14.9 4.0 0.0 3.1 74.7 0.0 3.3 100 0.79 28 55 12 5 100 366 97.4 78 Example 14.9 3.5 0.0 3.5 70.3 4.4 3.3 100 0.81 28 55 12 5 100 366 97.4 79 Example 14.6 3.4 0.0 3.4 68.8 4.3 2.2 3.2 100 0.81 33 44 12 7 4 100 364 97.1 80 Example 4.7 8.1 7.0 0.0 3.5 72.1 2.3 0.0 2.3 100 0.24 100 100 330 97 81

As shown in Table 6, when the hydrogen storage alloys containing the crystal phase having the Ce₅Co₁₉ type crystal structure and having a composition defined as La_(h)R6_(i)R7_(j)Mg_(k)R8_(m) (wherein R6 is one or more kind elements selected from a group consisting of rare earth metals including Y and excluding La; R7 is one or more kind elements selected from a group consisting of Zr, Ti, Zn, Sn and V; R8 is one or more kind elements selected from a group consisting of Ni, Co, Mn, Al, Cu, Fe, Cr, and Si; and h, i, j, k and m satisfy 0≦j≦0.65; 2≦k≦5.5; 0.70≦h/(h+0≦0.85; and h+i+j+k+m=100), that is, the hydrogen storage alloy of Examples 44 to 46, Examples 54 to 60, Examples 62 to 64, and Examples 66 to 80, were used, it was confirmed that excellent discharge capacity and cycle performance were exhibited.

Examples 82 to 91

Using the alloy raw materials of the compositions shown in the following Table 7, hydrogen storage alloys of Examples 82 to 91 were produced in the same manner as Example 1. When the lattice images of these hydrogen storage alloys were observed by a transmission electron microscope (TEM), it was confirmed that crystal phases with different crystal structures were layered in the c-axis direction.

Further, with respect to the hydrogen storage alloys, the production ratios of the crystal phases were calculated and cycle performance measurement was carried out as same as Example 1. The results are shown together in Table 7.

TABLE 7 Capacity retention Composition ratio Produced phases La Pr Nd Y Mg Ni Co Mn Al (%) CaCu5 Ce2Ni7 Gd2Co7 Ce5Co19 Pr5Co19 La5MgNi24 AuBe5 Example 17.0 0.0 0.0 0.0 4.3 68.1 6.4 0.0 4.3 87.5 36.9 0.0 0.0 37.7 14.9 0.0 10.5 82 Example 17.9 0.0 0.0 0.0 3.4 68.1 6.4 2.1 2.1 88.6 25.9 40.3 0.0 24.9 5.9 0.0 3.0 83 Example 17.0 0.0 0.0 0.0 4.3 67.0 6.4 2.1 3.2 88.2 35.2 12.8 0.0 36.2 9.7 0.0 6.0 84 Example 14.9 0.0 0.0 2.1 4.3 68.1 6.4 2.1 2.1 97.9 1.2 8.7 0.0 23.7 53.1 13.4 0.0 85 Example 12.8 0.0 5.1 0.0 3.4 67.0 8.5 0.0 3.2 96.0 2.9 13.1 0.0 17.0 67.0 0.0 0.0 86 Example 12.6 4.2 0.0 0.0 4.2 69.5 6.3 0.6 2.5 96.0 9.8 0.0 0.0 22.2 6.9 61.1 0.0 87 Example 12.8 4.3 0.0 0.0 4.3 68.1 6.4 0.0 4.3 94.2 11.8 0.0 0.0 69.1 9.0 7.1 3.0 88 Example 12.8 4.3 0.0 0.0 4.3 61.7 12.8 0.0 4.3 92.5 14.1 0.0 0.0 60.1 10.1 10.0 5.7 89 Example 12.4 4.1 0.0 0.0 4.1 69.1 6.2 2.1 2.1 91.6 21.7 42.9 0.0 19.7 15.7 0.0 0.0 90 Example 9.4 7.3 0.0 0.0 4.2 68.8 6.3 2.1 2.1 91.7 22.0 39.8 13.4 24.9 0.0 0.0 0.0 91

Further, with respect to the hydrogen storage alloys, based on the results of Table 7, a graph showing the relation of the capacity retention ratio to the production ratio of CaCu₅ phase is shown in FIG. 8.

As shown in FIG. 8, in the case of using the hydrogen storage alloys with 22% by weight or less of CaCu₅ phase, that is, the hydrogen storage alloys of Examples 85 to 91, it was confirmed that the capacity retention ratios were further higher values and particularly, in the case the CaCu₅ phase was 5% by weight or less, it was confirmed that the capacity retention ratios became extremely high values.

Examples 92 to 101

Using the alloy raw materials of the compositions shown in the following Table 8, hydrogen storage alloys of Examples 92 to 101 were produced in the same manner as Example 1. When the lattice images of these hydrogen storage alloys were observed by a transmission electron microscope (TEM), it was confirmed that crystal phases with different crystal structures were layered in the c-axis direction.

Further, with respect to the hydrogen storage alloys, the production ratios of the crystal phases were calculated in the same manner as Example 1. Furthermore, with respect to the hydrogen storage alloys, using Siebert PCT measurement apparatus (manufactured by Suzuki Syokan Co. Ltd., P73-07), the equilibrium pressure at 80° C. in case of H/M=0.5 of PCT curve (pressure-composition isothermal curve) was calculated. Further, after cells using the respective hydrogen storage alloys were left at 45° C. for 14 days, the remaining discharge capacity was measured in the same manner as described above and the remaining discharge capacity to the maximum discharge capacity was calculated. The results are also shown in Table 8.

TABLE 8 Equilibrium Remaining pressure capacity Composition (Mpa) (%) La Pr Nd Y Mg Ni Co Mn Al B/A Example 92 0.035 17.0 0.0 0.0 0.0 4.3 68.1 6.4 1.7 2.6 3.7 Example 93 0.045 73.51 17.0 0.0 0.0 0.0 4.3 70.2 6.4 1.1 1.1 3.7 Example 94 0.052 75.17 17.0 0.0 0.0 0.0 4.3 72.3 4.3 2.1 0.0 3.7 Example 95 0.064 74.32 12.8 4.3 0.0 0.0 4.3 71.3 4.3 3.2 0.0 3.7 Example 96 0.057 76.21 16.7 0.0 0.0 0.0 4.2 68.8 6.3 1.7 2.5 3.8 Example 97 0.047 74.98 12.8 4.3 0.0 0.0 4.3 68.1 6.4 1.7 2.6 3.7 Example 98 0.11 70.72 5.0 13.8 0.0 0.0 2.1 77.1 0.0 0.4 1.7 3.8 Example 99 0.13 69.11 12.9 0.0 0.0 4.3 4.3 77.4 0.0 0.0 1.1 3.65 Example 100 0.18 68.45 4.3 0.0 13.2 0.0 3.8 68.1 6.4 1.1 3.2 3.7 Example 101 0.068 71.33 8.4 8.4 0.0 0.0 4.2 69.5 4.2 5.3 0.0 3.75 Produced phases Ce2Ni7 Gd2Co7 Ce5Co19 Pr5Co19 CaCu5 AuBe5 La5MgNi24 Total Example 92 14.59 0 14.47 9.84 8.03 0 53.06 100.0 Example 93 39.03 0 40.22 8.37 12.38 0 0 100.0 Example 94 36.3 0 32.6 18.4 12.7 0 0 100.0 Example 95 44.47 4.04 28.92 16.29 6.27 0 0 100.0 Example 96 14.39 0 18.53 8.62 16.34 0 42.12 100.0 Example 97 17.88 0 24.38 48.42 6 0 3.33 100.0 Example 98 30.22 0 39.67 13.89 16.23 0 0 100.0 Example 99 13.05 33.51 43.44 0. 10.02 0 0 100.0 Example 100 36.1 0 33.45 8.81 18.9 2.77 0 100.0 Example 101 20.54 0 30.23 29.64 19.6 0 0 100.0

Further, with respect to the respective hydrogen storage alloys, based on the results in Table 8, a graph showing the relation of the remaining discharge capacity to the hydrogen equilibrium pressure is shown in FIG. 9.

As shown in FIG. 9, in the case of using the hydrogen storage alloys with 22% by weight or less of CaCu₅ phase and having hydrogen equilibrium pressure of 0.07 MPa or lower, that is, the hydrogen storage alloys of Examples 92 to 97, it was confirmed that the remaining discharge capacity was high value.

Examples 102 to 109

Using the alloy raw materials of the compositions shown in the following Table 9, hydrogen storage alloys of Examples 102 to 109 were produced in the same manner as Example 1. Herein, in Example 102 and Example 108, respectively high purity materials were used for sources of La, Ce, and Nd and in Examples 103 to 107 and Example 109 excluding the former, misch metal including La, Ce, Pr, and Nd was used. When the lattice images of these hydrogen storage alloys were observed by a transmission electron microscope (TEM), it was confirmed that crystal phases with different crystal structures were layered in the c-axis direction.

Next, using these hydrogen storage alloys for negative electrodes, sealed cells were respectively produced and the cycle life was measured for each sealed cell. The practical procedure was as described below.

<Production of Negative Electrode>

An aqueous solution in which a thickener (methyl cellulose) was dissolved and each hydrogen storage alloy powder were mixed and further mixed with 0.8% by weight of a binder (styrene-butadiene rubber) to obtain a paste which was applied to both faces of a punched steel plate (thickness 35 μm) and dried and thereafter the resulting steel plate was pressed to a prescribed thickness (0.3 mm) to obtain a negative electrode.

<Production of Positive Electrode>

An aqueous solution in which a thickener (carboxymethyl cellulose) was dissolved and paste of an active material were packed in a foamed nickel substrate and dried and thereafter, the resulting substrate was pressed to a prescribed thickness (0.78 mm) to obtain a positive electrode. A material used as the active material was a material obtained by coating the surface of nickel hydroxide containing 3% by weight of zinc and 0.5% by weight of cobalt in form of a solid solution with 6% by weight of cobalt hydroxide.

<Production of Sealed Cell>

A jelly roll was produced by spirally rolling the obtained positive electrode and negative electrode at a ratio of positive electrode capacity 1 to negative electrode capacity 1.25 while inserting a separator between them and a positive electrode terminal part and a current collection terminal were resistance welded and thereafter, the jelly roll was housed in a cylindrical metal case. Further, 1.81 ml of an electrolyte solution containing 8 mol/L KOH and 0.8 mol/L LiOH was injected and a cover made of a metal and equipped with a safety valve was used for closing to produce each sealed cell with 2500 mAh AA size.

<Cycle Test>

After the above-mentioned sealed cell was initially charged at 20° C. and 0.02 It (A) (50 mA) for 10 hours, the cell was again charged at 0.25 It (A) (625 mA) for 5 hours. Thereafter, discharging at 20° C. and 0.2 It (A) (500 mA) to cut off voltage of 1 V and charging at 20° C. and 0.2 It (A) (500 mA) for 6 hours were repeated 10 times and finally discharging was carried out for chemical conversion treatment.

Thereafter, charging in condition of 0.5 It (A) and −dV=5 mV, 30 minute pause, and discharging (20° C.) at 1 It (A) to final voltage of 1V were repeated and the number of cycles when the discharge capacity became 50% of the initial capacity was defined as the cycle life.

The measurement results are shown in Table 9 and FIG. 10.

TABLE 9 Total ratio of Pr₅Co₁₉ phase, Ce₅Co₁₉ phase, Alloy composition (mol %) and Ce₂Ni₇ phase La Ce Pr Nd Mg Ni Co Al (% by weight) Cycle life Example 102 14.1 0.0 0.0 4.3 3.3 72.8 2.2 3.3 98 350 Example 103 15.2 0.4 0.9 2.0 3.3 72.8 2.2 3.3 98 350 Example 104 14.8 0.7 0.9 2.2 3.3 72.8 2.2 3.3 98 350 Example 105 14.6 0.9 0.7 2.2 3.5 72.8 2.2 3.3 96 340 Example 106 14.8 1.3 0.4 2.0 3.3 72.8 2.2 3.3 95 280 Example 107 13.9 2.2 0.2 2.0 3.5 72.8 2.2 3.3 85 150 Example 108 13.9 4.3 0.0 0.0 3.5 72.8 2.2 3.3 76 50 Example 109 16.3 0.4 0.2 1.3 3.5 72.8 2.2 3.3 88 160

As shown in Table 9, with respect to the hydrogen storage alloys of the invention, it was understood that even if misch metal was used as a raw material, the cycle life could be maintained for a relatively long time by controlling the Ce content to be 2.2 mol % or lower. Particularly, in the case of Example 103 to Example 106 in which the Ce content was 1.3 mol % or lower and the total ratio of Pr₅Co₁₉ phase, Ce₅Co₁₉ phase, and Ce₂Ni₇ phase was 95% by weight or higher were found having extremely excellent cycle life, similarly to the hydrogen storage alloy of Example 102 containing a high purity material, Nd, in a relatively high amount. 

The invention claimed is:
 1. A hydrogen storage alloy containing two or more crystal phases having different crystal structures, wherein the two or more crystal phases are layered in the c-axis direction of the crystal structures and the hydrogen storage alloy has a composition defined by a general formula R1_(d)R2_(e)R4_(f)R5_(g) (wherein R1 is one or more kind elements selected from the group consisting of rare earth metals including Y; R2 is one or more kind elements selected from the group consisting of Mg, Ca, Sr, and Ba; R4 is one or more kind elements selected from the group consisting of Ni, Co, Cr, Fe, Cu, Zn, Si, Sn, V, Nb, Ta, Ti, Zr, and Hf; R5 is one or two elements selected from Mn and Al; and d, e, f, and g satisfy 8≦d≦19; 2≦e≦9; 73≦f≦79; 1≦g≦4; and d +e+f+g=100) and satisfies 3.53≦(B/A)≦3.80 and 0.0593(B/A)+1.59≦rA≦0.0063(B/A)+1.81 in the case (B/A) is defined as (f+g)/(d+e) and rA (Å) is defined as the average atomic radius of R1 and R2, wherein the two or more crystal phases layered in the c-axis direction of the crystal structures comprise a crystal phase having Pr₅Co_(l9) type crystal structure and a crystal phase having Ce₅Co₁₉ type crystal structure.
 2. The hydrogen storage alloy according to claim 1, wherein the R1 is one or more kind elements R1′ selected from the group consisting of Ce, Pr, Nd, Sm, and Y and La at La/R1′ ratio of 5 or less; R2 is Mg; the R4 is one or two elements selected from Ni and Co; the R5 is Al; and the d, e, f, and g satisfy 16≦d≦19; 2≦e≦5; 73≦f≦78; and 2≦g≦4. 